Disturbances in public transport are an important issue for passengers, public transport operators and infrastructure managers. After the occurrence of large disturbances, there is often a strong call from passengers and society to make the public transport network less vulnerable – and therefore more robust – against these types of events. Despite the mentioned importance of considering robustness, the next limitations can be formulated regarding the way robustness of public transport networks is currently considered:When evaluating and improving robustness of public transport networks against large non-recurrent disturbances, a passenger perspective is not included to its full extent. There is a strong focus on independent network levels operated by a single public transport operator, instead of considering the integral, multi-level public transport network available for passengers.

In general, limited quantitative data is available about disturbances which occur on multi-level public transport networks and about the effects of these disturbances on passengers. Also there is limited knowledge about the robustness performances of different network levels relative to each other. Given these limitations, the following main research question is formulated:What methodology can be developed to evaluate the robustness of multi-level public transport networks and to evaluate robustness effects of measures for the case study network between Rotterdam and The Hague?

In this study, robustness is related only to major discrete events: large, non-recurrent events which affect infrastructure availability. In line with this, the next definition of robustness is used in this study:
‘Robustness is the extent to which the network is able to maintain the function it was originally designed for under circumstances which strongly deviate from plan’.
In this study, a methodology is developed which enables the evaluation of the current robustness of multi-level public transport networks, as well as the evaluation of proposed robustness measures. The case study shows that it is worth to consider another network level as back-up in case a certain network level is blocked. The result of the case study indicates that from a societal point of view, there is still room to improve the robustness of multi-level public transport networks.
The developed methodology can especially be developed further by incorporating en-route route choice possibilities in the transit assignment model. Further research is recommended especially to gain more knowledge about the behaviour of passengers in case they are confronted with major discrete events and in case they are confronted with crowded vehicles.